An organic electroluminescent element (hereafter, it is also called as an organic EL element) is a light-emitting element having a constitution in which an organic functional layer containing a luminescent organic compound is interposed between a cathode and an anode. A hole injected from an anode and an electron injected from a cathode by applying an electric field are recombined in a light-emitting layer to form an exciton. It uses light (fluorescence and phosphorescence) emitted when the above exciton is deactivated.
Organic materials used in the organic functional layer are generally insulating materials which are difficult to electrify due to the small charge mobility. However, an organic EL element can emit light by using organic materials with a thickness of about submicron with applying a voltage of several volts to several ten volts. This can be achieved by making use of the fact that when an injected charge density becomes larger than the charge density of the inside of the thin film, electrons will be passed by space-charge limited current not by ohm current. This space-charge limited current has a property to be inversely proportional to the cube of the film thickness, and to be proportional to the square of the applied voltage. Therefore, the electric current can be passed by using a thinned organic material. When a voltage is applied to a thin film having about a 100 nm, an electric current of several mA is passed. This means that an electric current can be passed through an organic material which is an insulator by making a sufficient thin film.
The charge transfer in the organic material is largely affected by the crystalline condition of the organic material. When an organic molecule is in a single crystalline state, it will be produced energy bands formed by a π-π interaction between the molecules. As a result, the mobility of the charge becomes large and an electric current will easily flow. However, when an organic molecule is made into a thin film of submicron order, which is required for producing an organic EL element, it is very hard to make a single crystalline. Therefore, it is used an amorphous film having no order of molecular arrangement for producing an organic EL element.
A technology development of high luminescence with small power consumption is required for realization of an organic EL element employing a thin amorphous film. It was reported an organic EL element using a phosphorescent material from an excited triplet state by researchers in Princeton University: M. A. Baldo et al., Nature, Vol. 395, pp. 151 to 154 (1998). Since then, active researches have been made for the materials emitting phosphorescence at room temperature: see M. A. Baldo et al., Nature, Vol. 403, No. 17, pp. 750 to 753 (2000), and U.S. Pat. No. 6,097,147.
In an organic EL element using a phosphorescent material, it is an important technical problem to control the recombination location of the exciton, and in particular, to make recombination of the exciton inside of the light-emitting layer for improving efficiency and lifetime of the element.
In recent years, it was said that it was important to control the orientation condition of molecules and to control the electric property and the optical property of the organic EL element for the purpose of controlling the recombination location of the exciton, and in particular, to make recombination of the exciton inside of the light-emitting layer of an organic EL element using an amorphous thin film. Because it is considered that the organic molecules in the amorphous thin film change their orientation to the electric field direction at application of voltage, and the mobility of the charge and the recombination rate will be changed.
Patent document 1 (JP-A No. 2013-26300) describes a method of improving light emission efficiency of an organic EL element. The method focuses on the organic molecule contained in the light-emitting layer located between the cathode and the anode. The method arranges a molecular structure of the organic molecule so that the transition dipole moment of the organic molecule will have a direction to the molecular plane, and controls the organic molecule to be the electric field direction (the vertical direction to the both electrodes) to which organic molecule is orientated during application of voltage. By this, all energy of the excited organic molecule will be converted to the visible light without being consumed for non-light-emitting recombination other than surface plasmon polariton excitation. However, since the organic molecules are mutually affected with the transition dipole moment during application of voltage, the organic molecules will form aggregation. Therefore, it is difficult to control the orientation change of the molecule during application of voltage.